Why does your 1989 Jeep Cherokee fail the nox part on emissions?

A 1989 Jeep Cherokee wouldn't fail a NOx (nitrogen oxides) emissions test because NOx emissions standards weren't rigorously enforced for vehicles of that era. The catalytic converter technology needed to effectively reduce NOx to acceptable levels wasn't widely mandated or as effective as it is in modern vehicles. Cars from 1989 generally only had to meet hydrocarbon (HC), carbon monoxide (CO), and sometimes carbon dioxide (CO2) emissions standards. A failure on a modern emissions test for a '89 Cherokee would likely be due to issues with HC and CO, not NOx.